Plz explain knee jerk reflex.

Dear Student, When a hammer is hit on knee then a reflex occurs due to sensation carried via sensory nerves and then motor nerves carry the impulse to muscles that result in contraction of muscles and hence there is a jerk in knee. This process is known as knee jerk reflex. This is an important test used to test the functioning of muscles and nerves of knee. It is also known as patellar reflex. It tests L2, L3 and L4 segments of spinal cord. Regards